1. Field of the Invention
This invention relates to a process and an installation for separation of magnetic particles in a fluid for biological analysis of rare occurrences.
2. Discussion of the Background
The invention finds its application more specifically in the areas of medical diagnosis and quality control particularly in the agro-foodstuffs industry, but also in the therapeutic area in order to detect and extract a cell type with low occurrence in samples.
There may be cited by way of examples of medical applications of the invention:
The separation of fetal cells occurring in the maternal blood. In this case, it involves selectively collecting cells occurring in a ratio of approximately 1 fetal cell to 10 million non-fetal cells. The use of this invention should make it possible to replace the risky extractions of amniotic cells.
The preparation of stock cells of bone marrow, making it possible to resow marrow destroyed following anti-cancer treatment by chemotherapy or radiotherapy. As in the case of fetal cells, the number of bone-marrow cells in the peripheral blood is far below that of other cells, on the order of 1 to 200,000.
The early detection of circulating cancerous cells or micrometastasis, which is of the greatest value for determining exploratory and therapeutic strategies. There again, the objective is to isolate and identify 1 cell in approximately 5 million nucleate cells.
In the area of biological monitoring of foods or the environment, the objective is to obtain a rapid result without going through the conventional phase of propagation in a culture. The expected sensitivities are on the order of 1 cell (bacterium, yeast or mold) to 1 gram or even 10 grams of product. In the prior art there are many devices making it possible to reach this goal. Among the latter may be cited:
Physical methods based on differences of size, density or electrical charge (De Duve, 1971; Zeiller, 1972; Pretlow and Pretlow, 1982). But these methods show a lack of specificity.
Immuno-affinity methods consisting in affixing an antibody on a carrier, which antibody reacts with respect to an antigenic motif present on the surface of the cells sought (Forsgren and Sjoquist, 1986; Langone, 1982). Various processes deriving from the immuno-affinity methods, such as affinity chromatography (Hunt et al., 1982), have been proposed.
Cellular separation combined with a fluorescence detection deriving from the so-called "FACS" method for the English term "Fluorescence Activated Cell Sorting." This widely described method makes use of sophisticated equipment comprising a liquid flux in which the cells move. A laser beam stimulates the fluorescence and thus sets off a signal which makes it possible to divert the cell electrically into a container. This method if very effective and makes it possible to achieve an enrichment of nearly 100%, but it is not suitable for the sorting of large populations.
The magnetic separation methods designated as "MACS" for the English term "Magnetic Affinity Cell Sorting," which is based on the use of magnetic particles ingested by the cells (Melville et al., 1975) or selectively affixed to the cells through the expedient of antibodies (Molday et al., 1977).
The FACS and MACS methods are well known nowadays and already have resulted in an industrial implementation. Generally speaking, the results obtained with each of these methods differ little in terms of efficacy and make possible between 70 and 100% recovery. Nonetheless, they are more or less suitable to the various applications contemplated.